RELATIVE GRAVITY MEASUREMENTS

Transcription

1 RELATIVE GRAVITY MEASUREMENTS IN ILULISSAT IN JULY 2013 Final Report June 2015 Prof. Dr. Olivier Francis University of Luxembourg Faculty of Sciences, Technology and Communication Campus Kirchberg 6, rue Coudenhove-Kalergi L-1359 Luxembourg Grand-Duché de Luxembourg Tel. : , olivier.francis@uni.lu

2 Foreword This report contains the results of relative gravity measurements carried out in Ilulissat (Greenland) in July It was done in conjunction with the first absolute gravity measurements at the Ilulissat airport with the FG5X-216 from the University of Luxembourg. This station was established by Emile Nielsen from the Technical University of Denmark (DTU) in 2009 and repeated in 2010 with the absolute gravimeter A René Forsberg kindly provided us with the gravity station descriptions from the National Gravity Net. We present the results of the vertical gravity gradient at the absolute gravity station and the gravity ties from that station to four stations from the National Gravity Net. We are grateful to René Forsberg and Emile for their kind and friendly cooperation. 1. Vertical Gravity Gradient The vertical gravity gradient was measured at the absolute gravity site in the airport hangar. We measured the gravity differences between three different levels (0.260 m, m and m) above the benchmark (Figure 1). We obtained a linear vertical gravity gradient of / microgal/cm. Figure 1. Vertical gravity gradient measurements with the Scintrex at the absolute gravity station in the airport hangar in Ilulissat. 1

3 2. Relative measurements Relative gravity measurements were carried out with the Scintrex CG5-010 from the University of Luxembourg. The base station of the network is the new absolute gravity station at the airport, named Four gravity stations (58201, 58801, and 58803) from the National Gravity Net were visited (see Figures 2 and descriptions in the Annexes). It was important to find the gravity station This station was measured with the Jilag-3 absolute gravimeter in May by the Hannover team (Timmen et al., 2008). Measuring the gravity difference between the new and the old absolute gravity stations allow us to connect the absolute measurements from 1988 with the ones of The building was still standing. However, the floor has been covered with laminated parquet just a few years ago. The room is now used as a canteen. One of the employee confirmed that the marker was left in place. We estimated the location of the marker based on the information we had (see Annexes). We added 2 cm to our reference height to take the thickness of the parquet into account. In addition, the gravity point close to the permanent GPS station along the runway at the airport was also measured (named ). Unfortunately, the gravity station at the Ilulissat hotel was not found due to recent remodelling of the hotel. Station 00001: absolute gravity station at the Ilulissat Airport 2

4 Station 00002: Permanent GPS at the Ilulissat Airport. Station 58201: Ilulissat Churh. 3

5 Station 58801: Ilulissat Airport. Station 58802: old absolute gravity station. 4

6 Station 58803: old absolute gravity backup station. Figures 2. Gravity stations of the relative network measured in Ilulissat. 5

7 Figure 3. Locations of the gravity network sites measured in Ilulissat. 6

8 First, we measured the gravity differences from the new absolute gravity station and the stations and in the airport area. Then, we measured the differences between the station and stations 58802, and The results are presented in table 1. Table 1. Measured relative gravity ties in Ilulissat in microgal. Station ± ± ± ± ± 1.6 We can then calculate the gravity differences of each station with respect to the absolute gravity station (Table 2). Table 2. Relative ties in Ilulissat with respect to the new absolute gravity station at the airport. Site Code Lat Long Altitude /m g /microgal RMS microgal New absolute site at the airport Permanent GPS site at the airport Church Airport Old absolute gravity site Old absolute gravity backup site In 2013, we found a gravity difference between the stations and of /- 5.1 microgal. It matches perfectly the value of /- 7 microgal of Now, we can use the absolute gravity measurement value obtained at the station in 2013 to deduce the gravity values at the other stations (Table 3). Table 3. Gravity values at the National Gravity Net stations in Ilulissat. Station g /microgal Uncertainty /microgal

9 Conclusions In July 2013, four National Gravity Net stations from Ilulissat have been remeasured with a direct connection to the new absolute gravity station at the airport. The derived absolute gravity values have been calculated at those stations. Interestingly, we found the same gravity difference (at the microgal level) between the old absolute gravity station and its backup site as measured in References Nielsen J. E., Absolute gravimetry - for monitoring climate change and geodynamics in Greenland, Ph. D. Thesis, DTU Space, (ISBN: ), pages: 152, Timmen, L., Gitlein, O., Müller, J., Strykowski, G. and R. Forsberg, Absolute gravimetry with the Hannover meters JILAg-3 and FG5-220, and their deployment in a Danish- German cooperation, Zeitschrift für Vermessungswesen (zfv), Heft 3/2008, 133. Jahrgang, S ,

10 ANNEXES Gravity station descriptions 9

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